1. Field of the Invention
The present invention relates generally to air-fuel control arrangements and more specifically to a diagnostic arrangement which monitors the functioning of an exhaust gas oxygen sensor and enables a malfunction of the same to be detected.
2. Description of the Prior Art
In order to accurately maintain the desired air-fuel ratio it is common to sense the content of the exhaust gases (i.e. the oxygen concentration) and to perform feedback control of the fuel supply in order to obtain said desired air-fuel ratio.
Commonly used systems usually include a zirconia type sensor and use the level of the output of the sensor for air-fuel ratio feedback control. However, in the event that the sensor malfunctions the appropriate supply of fuel required to maintain the desired combustion characteristics becomes impossible and leads to the situation wherein the engine emissions increase and/or misfiring tends to occur. However, even under these conditions it is difficult for the average driver to diganose the problem accurately.
In order to overcome this problem arrangements of the nature disclosed in JP-A-57-72054 have been proposed wherein a memory is provided in the control circuit and arranged to contain pre-recorded data which enables the ready detection of a malfunction. This arrangement detects if the sensor output fluctuates back and forth between a level which is indicative of a rich mixture and that indicative of a lean one in order to determine the presence or absence of a malfunction. However, as the sensor is incapable of generating a suitable signal until properly warmedup operability thereof is delayed until such time as the engine has just completed its warming up.
However, if the lead line connecting the sensor to the control system breaks, it in fact becomes impossible to actually determine if the sensor is malfunctioning or not.